Currently, antibody drugs are attracting attention as the most reliable molecular targeted drugs, making a contribution to the rapid expansion of a new pharmaceutical field. The majority of the antibody drugs currently under development or in use employ antibodies belonging to the immunoglobulin G (hereinbelow, denoted as “IgG”) class.
Conventionally, protein such as Staphylococcus aureus-derived protein A or protein G is used for the purification of IgG antibodies (Non Patent Literatures 1 and 2). Since these proteins also bind mouse IgG as well as rabbit IgG, they have been popularly used for the purification of IgG at a laboratory reagent level. However, in light of the recent trend of the use of antibody drugs, particularly human IgG1, in the pharmaceutical field, the importance of industrial and pharmaceutical application of these proteins has been increasing more than ever. Particularly, protein A columns play a central role also in the purification of antibody drugs, and many antibody drug manufacturers have adopted a protein A column-centered purification system.
However, some problems associated with protein A columns are pointed out. One problem is the contamination of purified antibodies with protein A. Because protein A is a protein derived from bacteria, it becomes highly immunogenic when administered to the human body. Also, endotoxin contamination is a concern. Thus, in order to prevent contamination with unfavorable substances, protein A is required to be highly purified as an affinity ligand used for the purification of pharmaceutical products. This causes an increase in the cost of protein A column used for the purification of pharmaceutical products.
In order to solve this problem, the development of a new purification system for IgG antibodies is ongoing. For example, protein A mimetic peptides (Non Patent Literatures 3 and 4) and a non-peptide affinity ligand (Non Patent Literature 5) that was designed based on a X-ray crystallographic structure formed between protein A and the Fc portion of an IgG antibody have been reported. However, their use was limited due to problems associated with their binding ability and specificity.
Also, a number of studies have been undertaken in search for a novel IgG-binding peptide using, for example, phage libraries and synthetic peptide libraries (Patent Literatures 1 to 3).
As described above, research of purification of IgG antibodies using a new peptide or small molecule has been ongoing; nevertheless, no new purification system applicable on an industrial scale that can substitute a protein A or G column has yet been available, and there is still a need for a new technique for the purification of IgG antibodies in this field.